Electroluminescence is the emission of light under electric-field excitation. Based on this mechanism, EL lamps are finding an increasing number of applications in the field of flat panel displays due to the growing demand for consumer electronic products, e.g., cellular phones and portable computing devices. EL lamps also provide uniform light emission independent of viewing angle and they are insensitive to mechanical shock and vibration. They can be easily DC-driven at 1.5-9 volts by using inverters that generate AC voltages of about 100-300 V (peak-to-peak) at frequencies of 50 to 1000 Hz.
The two major EL lamp constructions are generally referred to as thin film and thick film. Thin-film EL lamps are made by depositing alternating thin layers of dielectric materials, phosphors and conductive oxides on a glass substrate using a vapor deposition technique such as CVD. Thick-film lamps are made by suspending powdered materials in resinous materials and then applying the materials in layers onto a plastic film using conventional screen printing techniques. Hence, the thick-film EL lamps can be thin, flexible and rugged thereby making them suitable for a wider range of lighting applications.
The phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of these phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Typically, the individual particles of the EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150.
For signage and backlighting involving multi-color informational displays, it is important to be able to generate a white light with a high color rendering index (CRI). However, the brightest electroluminescent phosphors emit primarily in the blue to green spectral region at wavelengths from about 400 nm to about 550 nm. This makes it difficult to obtain a high CRI, white emission with EL lamps especially since a satisfactory red-emitting electroluminescent phosphor for thick-film EL lamps has yet to be developed.